The present invention relates to the following used in a system (apparatus/method) for playing back information recorded on an information medium (optical disk) by using a focused light beam (laser beam) and recording/playing back information on/from an information medium:
(01) aberration state detection of a focused light beam formed on the recording surface of an information medium;
(02) aberration correction control on a focused light beam formed on the recording surface of the information medium;
(03) improvements (rationalization/simplification) in an optical system and processing circuit system in an optical head or information playback apparatus/information recording/playback apparatus having the track deviation detection function of making a focused light beam on the recording surface of an information medium stably trace a recording track and an aberration correction function for the focused light beam;
(04) improvements (rationalization/simplification) in an optical system and processing circuit system in an optical head or information playback apparatus/information recording/playback apparatus having the function of improving the quality of a playback signal from an information medium (performing crosstalk cancellation between adjacent tracks on the information medium) and an aberration correction function for a focused light beam formed on the recording surface of the information medium;
(05) the physical properties (structure, size, characteristics, and the like) of an information medium suited to an increase in density; and
(06) a manufacturing management method associated with information media (e.g., single-sided single-/dual-layer optical disks), which is used to provide an information medium that can guarantee stable recording operation or stable playback operation.
From another viewpoint, the present invention relates to the following as well:
(11) a thickness servo system using optical aberrations;
(12) a system for detecting the thickness irregularity of the transparent protective layer of an information medium by using optical aberrations;
(13) an information medium having a single-sided single-layer information recording layer (read-only layer or read/write layer) protected by a transparent protective layer;
(14) an information medium having a single-sided dual-layer information recording layer (each layer formed as a read-only layer or read/write layer) protected by a transparent protective layer;
(15) an objective lens control method using optical aberrations, and an optical head using this method;
(16) an aberration state detection method and a tilt servo control using this method;
(17) an optical head having an aberration detection optical system and differential push-pull signal processing system, and a servo system using this optical head; and
(18) an optical head having an aberration detection optical system and crosstalk cancellation processing system, and a servo system using this optical head.
Recently, the NA value of the objective lens of an optical head used in an information playback apparatus or information recording/playback apparatus has been increased to reduce the spot size of a focused light beam so as to increase the recording density of an information medium. For example, the NA value of the objective lens of a CD optical head is set to 0.47; the NA value of an early MO drive, 0.55; and the NA value of a present-generation DVD apparatus, 0.6. An NA value of 0.65 or more is being studied in an information playback apparatus or information recording/playback apparatus used for a next-generation DVD.
If thickness irregularity occurs in the transparent layer, the amount of spherical aberration caused in a focused light beam on the recording surface increases approximately in proportion to the fourth power of an NA value. In an information playback apparatus or information recording/playback apparatus with an NA value of 0.65 or more, therefore, a new technique of “a correction function (thickness servo control) for the thickness irregularity of a transparent layer” has been required to correct spherical aberration caused by the thickness irregularity of the transparent layer.
In addition, since the technique of “the transparent layer thickness irregularity correction function (thickness servo)” has not been used in information playback apparatuses and information recording/playback apparatuses, “the residual deviation amount (the upper limit of an allowable residual deviation amount) required for stable operation in transparent layer thickness irregularity correction function (thickness servo)” is completely unknown information in the past servo techniques.
The technique of “the transparent layer thickness irregularity correction function (thickness servo)” has not been used in information playback apparatuses and information recording/playback apparatuses. For this reason, information about “an allowable range of the thickness irregularities of transparent layers which exist before correction” is completely unknown information at present.
In designing a servo circuit for a transparent layer thickness irregularity correction function (thickness servo control), both the “required residual deviation amount” and “the range of the thickness irregularities of transparent layers which exist before correction (the maximum width of transparent layer thickness irregularity in non-defective disks within specifications)” are essential information.
As described above, in the prior art, since the NA value is 0.6 or less, an aspheric single lens is often used as an objective lens used for an optical head in an information playback apparatus/information recording/playback apparatus (optical disk drive). If the NA value is increased to 0.65 or more to increase the recording density in a next-generation DVD, a single-lens structure cannot be used owing to a problem in terms of a manufacturing technique for objective lenses, and an objective lens structure constituted by two or more lenses is required.
With an NA of 0.65 or more, to ensure a positional shift margin between lenses and tilt margin, a transparent layer thinner than that in a conventional DVD, which is 0.6 mm thick, (for example, a 0.1-mm thick layer) is required. If the thickness of a transparent layer is greatly decreased from 0.6 mm in the prior art, the influence of the thickness irregularity of the transparent layer relatively increases. This makes it difficult to use the same manufacturing method (quality control method) as that for an information medium used for a conventional DVD.
For this reason, a next-generation DVD uses the following information medium structure. A substrate (which need not always be transparent) is placed in a direction (on the disk lower surface side) in which recording or playback light is shielded, a reflecting layer or recording layer is formed on the substrate, and a thin (for example, a nominal thickness of 0.1 mm) transparent protective layer is formed on the recording layer.
For an information medium (optical disk) for such a next-generation DVD, methods of forming a transparent layer that exists in a direction in which recording or playback light is transmitted may include, for example:
(A) a method of coating a reflecting or recording layer with a transparent layer by spin coating, blade coating, or the like; and
(B) a method of bonding a transparent layer (transparent sheet) formed in advance onto a reflecting layer or recording layer through a transparent adhesive layer.
It is impossible to measure the thickness of a transparent layer in the process of manufacturing an information medium by using either of methods (A) and (B) alone, and hence the thickness of the transparent layer must be measured upon completion of the information medium.
An information medium (optical disk) used for a conventional DVD has a structure in which a reflecting layer or recording layer is directly formed on a transparent substrate with a thickness of 0.6 mm, and the recording surface (reflecting layer or recording layer) is irradiated with a laser beam through the 0.6-mm thick transparent substrate. For this reason, the thickness irregularity of the transparent substrates of information media used for a conventional DVD can be easily managed by mechanically measuring the thickness of each transparent substrate by using a micrometer or the like before a reflecting layer or recording layer is formed in a manufacturing process. However, such a mechanical measurement method is not suited to measuring and managing the above thin (0.1 mm) transparent layer with high precision.
In information media whose compatibility among products must be guaranteed, the transparent layer thickness irregularity range allowed for information media (mass-produced optical disks) is defined in specifications. In defining a transparent layer thickness irregularity allowable range in specifications, however, any method of measuring the thickness of a transparent layer portion has not yet been established.
As a method of measuring the thickness of the transparent layer of a completed medium (optical disk), a method of peeling off part of a transparent layer and measuring a level difference in accordance with the presence/absence of a transparent layer by a turnery step. Since this method is a destructive measurement method for an information medium, the information medium cannot be used after the measurement (destruction of part of the transparent layer).
At present, no nondestructive measurement method is available, which can be used to measure the thickness of the thickness of a transparent layer in an information medium (optical disk) for a next-generation DVD, and hence a high-precision nondestructive measurement method is required.
In addition, there is no information as to what to do about the characteristics (e.g., the relationship between thickness and refractive index) of the transparent layer of an information medium (optical disk) whose transparent layer has thickness irregularity in an allowable range.
DVD-ROM disks that are currently on the market include single-sided dual-layer disks. According to the specifications for currently available DVD-ROM disks, the thickness of the portion between two reflecting layers is defined to 55±15 μm, and the distance from the surface (incident surface) of a DVD-ROM disk to each recording layer is defined within the range of +53 μm to −50 μm with respect to a reference value of 0.6 mm. The NA value of an optical head used in a currently available DVD is 0.60, and a thickness error range of +53 μm to −50 μm is included in an allowable aberration range in a playback optical system.
As described above, however, the amount of spherical aberration increases in proportion to the fourth power of the NA value of an objective lens. For this reason, if the NA value is increased to 0.65 or more to increase the recording density of an information medium, the amount of spherical aberration sharply increases, and the increased aberration amount falls out of the above allowable aberration range (+53 μm to −50 μm).
To solve this problem, the following two measures must be taken:                reducing the distance between the two layers to let the amount of spherical aberration fall within the allowable aberration range in the playback optical system; and        strictly managing the precision of the thickness from the disk surface (incident surface) to each layer, i.e., a reflecting surface or recording surface. In this case, however, the following problems arise:        If the distance between the two layers is reduced, light reflected by one reflecting layer or recording layer leaks into the photodetector while information is played back from the other reflecting layer or recording layer, resulting in interlayer crosstalk and a deterioration in playback signal.        If the precision of the thickness from the disk surface to each layer is strictly managed, the manufacturing yield of information media (optical disks) decreases, resulting in an increase in the sales price of information media.        
Consider a case where information is optically read from a high-density recording layer under a thin transparent layer (e.g., 0.1 mm thick) having thickness irregularity by using a laser beam of a short wavelength (e.g., 405 nm) focused by an objective lens having a large NA (e.g., 0.85). In this case, two servo operations are required: focusing servo operation of bringing a laser beam to a focus on the recording layer (focusing the beam into a minimum laser spot on the recording layer); and thickness servo operation of correcting an optical aberration caused by the thickness irregularity of the transparent layer (so as to prevent the laser spot size from being increased by an aberration even if the focus is automatically adjusted to a best point by focusing servo operation).
As a method of simultaneously performing focusing servo operation and thickness servo operation, a method of making a focusing servo optical system and thickness servo optical system wobble at different frequencies may be used. In this method, by detecting the envelope of a playback RF signal obtained from an optical disk through wobbling, focusing servo control and thickness servo control are performed (since the wobble frequency of the playback RF signal envelope in focusing servo control differs from that in thickness servo control, frequency separation of servo detection signals in the two servo operations can be performed).
In this method, however, much crosstalk occurs between a detection signal in focusing servo control and a detection signal in thickness servo control, and hence it is impossible to completely separate a focusing servo loop from a thickness servo loop. It is therefore difficult to provide sufficient, stable servo control, resulting in stricter requirement for the thickness irregularity allowable range for transparent layers. In addition, since the upper limit of servo response frequencies is restricted by a wobbling frequency, high-speed servo control is difficult to realize.
As described above, when an optical disk (information medium) tilts with respect to the objective lens due to the influence of warpage and the like in the radial direction and/or circumferential direction of the optical disk, coma occurs in a focused light beam on the recording surface. When this coma occurs, the spot size of the focused light beam on the recording surface increases to cause a deterioration in playback signal from the optical disk (information medium) or instability in recording on the optical disk (information medium) as in the case of the occurrence of spherical aberration.
To solve this problem, an additional tilt servo mechanism for correcting the influence of coma caused by “the tilt of a transparent layer with respect to the objective lens” upon the warpage of a disk (information medium) or the like is introduced (as needed) as well as an additional thickness servo mechanism for correcting the influence of spherical aberration caused by “the thickness irregularity of the transparent layer of a disk (information medium)”.
If, however, a thickness servo mechanism and/or tilt servo mechanism is additionally introduced, including their detection optical systems (in addition to the above DPP and/or CTC optical systems and circuit systems), the servo system (detection system and circuit system) is complicated accordingly, resulting in an increase in the cost of an optical head.